app: settings gained "disabled-sources" key

This key will keep a list of the uid of the sources marked as disabled
in the interface in order to keep the same state between runs of the application

data/org.gnome.calendar.gschema.xml.in

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
 <schemalist gettext-domain="gnome-calendar">
     <schema id="org.gnome.calendar" path="/org/gnome/calendar/">
         <key name="window-maximized" type="b">
@@ -20,5 +21,10 @@
             <_summary>Type of the active view</_summary>
             <_description>Type of the active window view, default value is: monthly view</_description>
         </key>
+        <key name="disabled-sources" type="as">
+            <default>[]</default>
+            <_summary>List of the disabled sources</_summary>
+            <_description>Sources disabled last time Calendar ran</_description>
+        </key>
     </schema>
 </schemalist>